The gut microbial diversity of killifish decreases with age just like humans. At 16 weeks of age which is the end of their lifespans, the fish that were transplanted with the young microbiome were as active as 6-week old fish.
The mechanism by which having a young microbiome increases lifespan is unclear but may have to do with reprogramming the immune system since gut bacteria are a major regulator of the immune system.
Whether microbiome transplants from young animals from other species such as mice have any effect on lifespan needs to be tested.